1. Field of the Invention
This invention relates to an automatic exposure control device suited for a video camera or the like.
2. Description of the Related Art
The image sensing apparatuses such as a video camera, an electronic camera and the like have been arranged to automatically control their exposure operations. In the case of the video camera, for example, an automatic exposure control device controls the gain of a video signal to keep its level constant by means of the iris of an optical system and an automatic gain control (hereinafter referred to as AGC) circuit.
FIG. 1 of the accompanying drawings is a block diagram showing in outline the arrangement of a typical video camera including the conventional automatic exposure control device. In FIG. 1, a reference numeral 1 denotes a photo-taking lens. The light quantity of incident light passing through the lens 1 is adjusted by an iris 2. The light then comes to an image sensor 3 which is, for example, a CCD or the like and forms an image on the imaging plane of the image sensor 3. A buffer amplifier 4 which is disposed on the output side of the image sensor 3 produces a video signal 41. The video signal 41 is supplied to an AGC circuit 5 and a light measuring circuit 7a. The AGC circuit 5 controls the gain of the input signal to keep its output signal 51 at a constant level. The video signal 51 output from the AGC circuit 5 is supplied to a light measuring circuit 7b and a signal processing circuit 8. The signal processing circuit 8 then performs a signal processing operation including gamma correction, a blanking process, addition of synchronizing signals, etc. to convert the video signal into a standardized TV signal which is suited for display by a monitor, a video recorder or the like which is not shown. The processed signal is supplied to an output terminal OUT.
Light measuring circuits 7a and 7b are arranged in the same manner. They detect the signal levels of the video signals 41 and 51 and produce outputs 71a and 71b respectively. The output 71a of the circuit 7a is supplied to an iris driving circuit 6 which is arranged to control the aperture position of the iris 2. The out-put 71b of the circuit 7b is supplied to the gain control terminal 5a of the AGC circuit 5. The aperture of the iris 2 and the gain of the AGC circuit 5 are thus control-led to keep the video signal level constant.
The light measuring circuits 7a and 7b have hitherto been arranged as averaging circuits in general. This arrangement makes constant the luminance of the whole image plane on the average. In the event of a great difference in luminance between an object to be photographed and the background thereof, therefore, the image of the object is darkened all over under a back light condition or is whitened all over under a normal light condition. The image then becomes unnatural.
To avoid this, a weighted light measuring method of measuring light by attaching weight to a part of the image plane has been proposed. FIG. 4 shows by way of example the weighted light measuring method. In FIG. 4, a reference numeral 200 denotes a whole image plane. A numeral 201 denotes an object to be photographed. A numeral 202 denotes a light measuring area (or a light measuring frame) which is a weighted light measuring part of the image plane. If the object is located within this light measuring area, an exposure can be effected always appositely to the object without being much affected by the background. Hereinafter, this light measuring method will be called a frame-weighted light measuring method.
FIGS. 2 and 3 show an example of a light measuring circuit arranged in accordance with the frame-weighted light measuring method. The illustrations include light measuring circuits 9a and 9b and a light measuring area signal generating circuit 10. With the exception of them, the rest of the circuit is identical with the circuit of FIG. 1. Within the light measuring circuit 9a, an analog switch 901 is arranged to act as gate means for causing only a video signal part corresponding to the inside of the light measuring area (or frame) 202 of FIG. 4 to pass. The circuit 9a further includes low-pass filters 902 and 903, resistors 904 and 905, and a buffer amplifier 906. A video signal 41 is arranged to be supplied to the low-pass filter 902 and also to the other low-pass filter 903 via the analog switch 901. The analog switch 901 operates under the control of a control signal 101. The control signal 101 is a light measuring area setting gate signal. The level of the control signal 101 becomes high for the inside of the light measuring area 202 of FIG. 4. This control signal 101 is generated by the light measuring area signal generating circuit 10 of FIG. 2 as an H-V composite signal which is in synchronism with a synchronizing signal SYNC. The light measuring area setting signal generating circuit 10 is arranged to receive the synchronizing signal SYNC, to generate the light measuring area setting signal 101 and to cause thereby only the video signal part corresponding to the inside of the light measuring area 202 to pass. The low-pass filter 902 is arranged to produce an average value signal for the whole image plane. The low-pass filter 903 is arranged to produce an average value signal for the inside of the light measuring area 202. The two signals output from the low-pass filters 902 and 903 are weighted and added by the resistors 904 and 905 and then output from the buffer amplifier 906. In this instance, if the weight attaching degree of the low-pass filter 902 is larger than that of the other low-pass filter 903, the light measurement is performed in a manner close to averaging the light of the whole image plane. If the weight attaching ratio between them is reversed, light measurement is performed with weight attached to the inside of the light measuring area 202.
In accordance with the frame-weighted light measuring method described above, the level of the video signal suddenly drops to darken the whole image plane when a high luminance light source or the like either enters the light measuring frame or passes through there. In addition to this, in the event of a back light condition, the video signal level greatly changes for a change in the background part included in the light measuring frame. Besides, the object's image readily darkens all over when the object's image is small relative to the light measuring frame. In the case of the video camera which is used for obtaining a motion picture, therefore, the image plane becomes unstable giving a very unnatural impression in these instances.
Further, in the frame-weighted light measuring method, the weight attaching process of adding the signal level of the light measuring area (light measuring frame) to that of the whole image plane is fixedly carried out. Therefore, this method gives the same weighted light measuring characteristic both in a case where the signal level of the light measuring area is higher than that of the area outside of the light measuring area (a forward light state) and in an opposite case (a back light state).
However, in order to ensure an adequate exposure by carrying out the frame-weighted light measurement, the additive weight attaching value to be used for a normal light condition generally differs from the value to be used for a back light condition. If the weight attaching value remains fixed, it would be difficult to have an adequate exposure both under the normal light and back light conditions, Therefore, it has been impossible to completely remove the unnaturalness of the image in accordance with the conventional light measuring method.
Prior patent applications related to exposure control of this kind include U.S. patent application Ser. No. 264,204 filed on Oct. 28, 1988 and U.S. Pat. Application Ser. No. 312,962 filed on Feb. 21, 1989.